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Homework Help: Bumper Car Collision Problem

  1. Oct 31, 2007 #1
    1. The problem statement, all variables and given/known data

    Walt and Wolfie collide in bumper cars of mass 50 kg each. Walt has a mass of 78 kg, and Wolfie has a mass of 61 kg. Walt strikes Wolfie from the rear at V = 3.7 m/s. If the collision is elastic, Wolfie is initially at rest, and Walt's final speed is 0.2655 m/s in the same direction, what is Wolfie's speed after the collision?

    2. Relevant equations

    3. The attempt at a solution
    I don't really know where to begin at... I don't think the equation above is the one I need but, I can't find any others. I know I don't use S=d/t. I also looked at the hyperphysics site but, the only equations for collisions and information on collisions that I could find were for head to head... Can anyone give me a push in the right direction? (no pun intended :p)
  2. jcsd
  3. Oct 31, 2007 #2


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    Collision is elastic.

    Conservation of momentum perhaps?
  4. Oct 31, 2007 #3
    err? I got that it was elastic..?
  5. Oct 31, 2007 #4


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    Well what do you know about elastic collisions? Also remember that momentum must be conserved.

    This collision is also er "head to head".With the information you are given the direction in which there are facing should have no outcome on the resulting velocites. Draw a diagram of before and after the collision and hopefully you'll see I mean.

    List your known variables, and see where that takes you . .
  6. Oct 31, 2007 #5
    I don't see what you mean? The variables are Walts total mass= 128 Kg. Wolfie total mass= 111 kg Walts velocity= 3.7 m/s and Walts end speed is .26555 m/s....
  7. Oct 31, 2007 #6


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    Ok now what do we actually know about collisions??

    First off, the momentum must be conserved, the sum of the momentums before equals the sum of the momentums after.

    Secondly our collision is perfectly elastic, even better still it is a straight-line elastic collision . .. actually it doesn't look like you'll need to even apply this.

    Just find the sums of the momentums before the interaction and equal them to the sums of the momentum after the interaction.
  8. Oct 31, 2007 #7


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    I'm guessing the problem is expected to be solved using conservation of kinetic energy since they specifically mentioned elastic collision(even though conservation of momentum will give the same answer)...

    I'm guessing Olivam, that you haven't covered momentum yet in your class?
  9. Oct 31, 2007 #8
    Conservation of energy statement to be used in this problem: (I am assuming, you are not supposed to use conservaion of linear momentum!)
    m1*(u1^2)/2 = m1*(v1^2)/2 + m2*(v2^2)/2
    where 1 & 2 refer to respectively, (Walt+car) & (Wolfie+car); u and v respectively refer to before-collision and after-collision situation.
  10. Oct 31, 2007 #9
    Ok! Thanks I got it now
  11. Nov 22, 2008 #10
    Can you please show your solution.
    I need this for my project and quite frankly, i don`t get anything.
    Please? :)
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